Projection apparatus



f March 16 1926.

F. TWYMAN PROJECTION APPARATUS Filed May 19, 1925 Afa,

INVENTORA FRANKTWYMAN.,

By his Attorneys, l aednm Patented Mar. 16, 1926.

UNITED lSTATES PATENT OFFICE.

FRANK TWYMAN,

PROJECTION Application filed May 19,

' 5 land, have invented new and useful Improvements '1n Projection Apparatus, of which the following is a specification.

This invention relates to projection ap paratus, particularly to projection kaleidol scopes which are sometimes termed decoroscopes. In such apparatus the central 1mage, that is the direct image of the object,` is successfully illuminated but the illumination of the other images, namely the virtual images of the object, is very poor.

It has been suggested to interpose a of ground glass near by this means the brightness of the outlying parts of the pattern is improved at the eX- pense of the brightness of the central part. The total loss of light however is so great as to preclude asuccessful projection of the image except for a very short distance, say 8 feet.

The object of thisinvention is to obtain the goed illumination of both the central and outlying parts of the pattern when .the picture isprojected a considerable distance, say 100 feet.

In apparatus with which the present invention is concerned a condensing lens is placed in front of the 'object to be proje :ted which may be a film or an object bo.; as used in kaleidoscopes and at the back of the Y film box is a prism whose cross section is a regular polygon'or mirrors which are coineident with the sides of such a geometrical prism. Light issuing from an electric lamp or other source passes through the condens- 60 ing lens and the film or box to the prism or mirrors and thence to. a projecting lens. By

this arrangement a picture is projected of the direct object and of the virtual images of the object, the word fobject including any line drawi coloured transparency,

pieces ofglass o I\the like, placed within the eiective space at' the object end of' vthe kaleidoscope prism, and` of which it is d esired to form a repeated pattern.

5 According to this invention I provide means for efficiently illuminating the pic'- tures on the screen, not only of the direct object, but ofthe virtual images of that obthe condensing lens;

OF LONDON, ENGLAND, AS'SIGNOR TO ADAM HILGER, LIMITED,

OF LONDON, ENGLAND.

APPARATUS.

1923. Serial No. 640,199.

ject. The efficient illumination of the picture of the direct object is effected by means of the condenser as in already existing apparatus. The efficient illumination ofthe picture of the virtual images 'is ensured by providing that rays of illuminating light fall upon the condenser at angles, such that the object is illuminated by rays in all the directions which light from it must take in 'order that the pictures of the virtual images may be illuminated bythe projecting lens on the screen.

This may be attained by an arrangement of optical elements which produce virtual images of the light source, the correct position for the said virtual images being at the foci of the condensing lens conjugate with the virtual images of the centre of4 the projecting lens formed by the kaleidoscope prism or mirror system. Or I may provide other sources of light located at the same conjugate foci.

In one embodiment. of the invention a similar prism, or similarly arranged mirrors, to those above referred to is placed between the source of light and the condensing lens. When there is a space between the first and second prisms (or mirrors) thesecond mentioned prism (or mirrors) `is larger in size than that first mentioned and it is found by calculation that when the cross section of the prism is a square or that when the mirrors vare arranged at right angles to each other in the form ofa square the dimensions of the prism (or mirrors) are ascertained from v the following formulae:

lzlength of projection prism or mirrors; 12=the same for light source whose image has widthdz;

wzwidth of same; L=length of illuminating prism or mirrors from a point of light source; Y

LZ1-.the same for a light source of width d; W=the width of the illuminating prism or mirrors; Y

i b-:the condenser. space, that is the space between the two prisms; this space in order 100 to reduce the size of the illuminating prism or mirrors should be as small as possible;

@ -the angular eld; j

namely the dif the 105 n=the number of images,

Samet-sita can] w (n n-Pwr.-

'Ihe particulars when prisms of other cross sections are used can be similarly calculated.

In the existing referred to there is lto the picture. It is projecting kaleidoscopes no sharply defined limit therefore either impossible to confine the picture to the limits of the screen or if the whole picture is small enough to appear on the screen the limiting margin of the entire picture is badly defined viflhereby a displeasing effect is produced on t e eye.

In order to overcome this objection I, according to this invention, limit the picture to a definite size and shape by inserting a second projecting lens and a diaphragm having in it an aperture between the end of the projecting prism or mirrors and the usual projecting lens. The second rojecting lens forms an image on the diap iragm of any desired `size and the light passes through the aperture to the 'ordinary projecting lens which projects the limited picture on to the screen.

shows diagrammatically a projection Y apparatus made in accordance with this invention vand Figure 2 shows a modiication of the same.

Figure 3 shows b and ci.

a is a light source cross sections of the lenses A from which light is admitted to a prism whose cross section is a square from which the light issues to a condensing lens e to another similar prism rl of smaller size than b, a space being left between the condensing lens c and the prism d for the interposition of the object such as lens, a prism whose l all of said L optical axis and optical elements W ich Q produce virtual images ofthe light source at the foci of the condensing lens, conjugate images f film lml to be projected and e is a projecting lens by which the image of the object is projected on to a screen. f are the virtual images of the light source a situated at the foci of the condensing lens c conjugate with the virtual images g on the centre of the projecting lens c formed by the prism d.

In the modification shown in Figure 2 another projecting lens 71. and a screen j having an aperture f: is inserted between the prism d and the projecting lens e.

In the following claims the term prism includes mirrors coincident with the side of a similar geometrical prism.

What I claim is 1. In projection apparatus, a projecting cross section is a regular polygon, a condenser and a light source, parts being arranged alon an with the virtual images of the center of the projecting lens formed by the prism.

2.. In projection apparatus, a projecting lens, 'a prism whose cross section 1s a regular parallelogram, a condenser and a light source, all of said parts being arranged along an optical axis and a prism similar to the other located between thelight source and the condenser.

3. In projection apparatus, a projecting lens, a prism whose cross section is a regular parallelogram, a condenser and a light source, all of said parts being arranged along an optical axis and a prism similar to but larger than the other prism located between the light source and the condenser so that a space is left between the prisms.

4. In 'projection apparatus, two projecting lenses and a screen having an aperture between the projecting lenses, a prism whose cross section is a regular polygon, a condenser and a light source, all of said parts being arranged along an optical axis and producing virtual images of the objects to be projected and means for illuminating the images on the screen corresponding to the virtual images formed by the prism.

5. In projection apparatus, two project- -ing lenses and a scale having an aperture In the accompanying drawing, Figure 1 b etween the two projecting lenses, a prism whose cross section is a regular polygon, a condenser and a light source, all of said parts being arranged along an optical axis and optical elements which produce virtual images o the light Source at the foci of the condensing lens conjugate with the virtual images of the center oi the projecting lens formed by the prism.

6. In projection apparatus, two projecting lenses anda Screen having an aperture between the projecting lenses, a prism whose cross section is a regular parallelogram, a condenser 'and a 'light source, all of said 130 parts being arranged along an o tical axis' axis and a prism similar to but larger than 10 and a prism similar to the other` ocated be-V the other prism located between the light tween the light source and the condenser. source and the condenser so that a space is 7. In projection apparatus, -two projectleft between the prisms.' i

5 ing lensesand a. screen havingan aperture In testimony that I claim the foregoing between. thel two projecting lenses, a prism as my invention I have signed my name this 16 whose cross section is a. regular paralleloeighth day of May 1923. gram, a condenser and a light source, all of v l said parts being arranged along an optical FRANK 

